已知A=3x2-4xy+2y2,B=x2+2xy-5y2.
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已知A=3x2-4xy+2y2,B=x2+2xy-5y2.
(1)求A+B;
(2)求A-B;
(3)若2A-B+C=0,求C.
(1)求A+B;
(2)求A-B;
(3)若2A-B+C=0,求C.
![已知A=3x2-4xy+2y2,B=x2+2xy-5y2.](/uploads/image/z/17922911-23-1.jpg?t=%E5%B7%B2%E7%9F%A5A%3D3x2-4xy%2B2y2%EF%BC%8CB%3Dx2%2B2xy-5y2%EF%BC%8E)
(1)∵A=3x2-4xy+2y2,B=x2+2xy-5y2,
∴A+B=(3x2-4xy+2y2)+(x2+2xy-5y2)=3x2-4xy+2y2+x2+2xy-5y2=4x2-2xy-3y2;
(2)∵A=3x2-4xy+2y2,B=x2+2xy-5y2,
∴A-B=(3x2-4xy+2y2)-(x2+2xy-5y2)=3x2-4xy+2y2-x2-2xy+5y2=2x2-6xy+7y2;
(3)∵2A-B+C=0,
∴C=B-2A=(x2+2xy-5y2)-2(3x2-4xy+2y2)=x2+2xy-5y2-6x2+8xy-4y2=-5x2+10xy-9y2
∴A+B=(3x2-4xy+2y2)+(x2+2xy-5y2)=3x2-4xy+2y2+x2+2xy-5y2=4x2-2xy-3y2;
(2)∵A=3x2-4xy+2y2,B=x2+2xy-5y2,
∴A-B=(3x2-4xy+2y2)-(x2+2xy-5y2)=3x2-4xy+2y2-x2-2xy+5y2=2x2-6xy+7y2;
(3)∵2A-B+C=0,
∴C=B-2A=(x2+2xy-5y2)-2(3x2-4xy+2y2)=x2+2xy-5y2-6x2+8xy-4y2=-5x2+10xy-9y2
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